In all transmission lines, the electrical parameters are determined by the physical dimensions and electrical properties of the components. It is common to have periodic or random variations in those properties that induce inconsistencies in the electrical transmission parameters. Within the frequency range of prior cable standards, several design approaches and prior art have been developed in an attempt in order to reduce the effect of those variations.
Insertion loss and return loss characteristics can have abrupt changes at specific frequencies that are related to the electrical wave interacting with the periodicity of the transmission line variations. If the signal wavelengths are sufficiently longer than the perturbations in the cable construction, then the effect of frequency dependent electrical parameters is much less evident or non-existent. However, if the signal wavelengths are in the same range as the cable perturbations, then the effect on the signal transmission is much more pronounced at those specific signal wavelengths that correlate with the dimension of the transmission line anomalies.
A particular source of anomalies in cable performance occurs when the pairs are assembled together. FIG. 1 shows the insertion loss results for a typical cable with non-shielded pairs and an overall metallic shield designed for 500 MHZ. A number of insertion loss electrical response anomalies exist below 2000 MHZ, and anomalies also occur in the return loss measurement results.
Another example is a cable with individually shielded pairs, with each pair being surrounded by a metal shield layer. The manufacturing processes cause periodic variations in spacing from conductor to conductor as well as spacing of the conductors to the pair shield. These variations cause anomalies in the insertion loss and return loss measurements as shown in FIG. 2.
The process of manufacturing a completed cable causes periodic mechanical perturbations. It is known that tension consistency and care in handling of the cable components is important. It was also discovered that one of the effects caused by the twisting action is the unintended spiral that is induced in the cable components by the twisting action. The spiral length is the same as the twist length. In rotating machinery, it is also common to have a slightly different path for the wire from the payoff spool through the machine as it turns. The periodic changing path in the wire results in having different sections of the components which bend and flex differently than other sections. The differences in bending lead to slight periodic differences in the mechanical structure in the cable and is one cause for anomalies in the electrical measurement results.
In FIG. 3, an insertion loss notch occurs at about 1.2 GHz while other insertion loss notches occur in the 2 GHz region and are due to the effect of the cabling process. Changing the process equipment can help reduce the notches (representing these insertion losses), but such changes in the process equipment do not completely eliminate the notches. The problems inherent in the process machinery, and thus the effects thereof still remain.
For shielded pairs, surrounding the pairs with a metallic tape is known to provide electrical isolation from one pair to the next pair. However, metallic tapes are generally not of sufficient tightness in order to provide the pair dimensional integrity to avoid electrical anomalies in the final cable test results. FIG. 4 shows the results of a cable with a metallic pair which is wrap with a wrap length of about 1 inch, but this arrangement does not provide the desired effect due to the relative looseness of the longer tape wrap. It is to appreciated that longer lengths of the wrap of the pair results in even less tightness and less mechanical integrity. A relatively short spiral length of the metallic tape over the twisted pair is needed to provide the necessary mechanical integrity for the wrapped pair.
However the metallic spiral shield wrap construction with a relatively short spiral alone was not found to provide the necessary shielding effectiveness. It was discovered that a combination shield could be employed such that a metallic tape wrap with a shorter lay length is applied over metallic wrap with a long lay length or in a longitudinal fashion. Note that a lay length is traditionally defined as the axial distance necessary for one pair of insulated conductors to complete a full 360 degree of rotation when twisting about one another, such that a tighter twist will result in a shorter lay length while a looser twist will result in a longer lay length. One arrangement is to have the conductive surface of the inner tape facing away from the pair and the shorter lay metallic tape with a metallic conductive surface on both sides to provide electrical contact with the inner longitudinal tape and to adjacent similar shielded pairs in the assembled cable.
FIG. 6 shows the improved crosstalk performance of a combination of two metallic shield tapes, compared to a single metallic wrapped tape with a short spiral, as shown in FIG. 5.
Cabling twist length can be chosen to be below about 0.5 wavelengths of the highest frequency of operation in order to move the cabling process and design electrical anomalies beyond the frequency of interest. However, at frequencies in the range of 2,000 MHZ, this approach has drawbacks due to the additional path length of the pairs within the shorter spiral length of each pair as well as a crushing action caused by the short lay lengths in the cable. This generally leads to problems in meeting specifications for cable propagation delay and insertion loss. However, with the design options provided by the pair wrapping, much longer cable lay lengths can be utilized, avoiding the problems caused by short cable lay lengths.
For the new extended frequency electrical requirements, the prior art does not solve all the problems found in designing and manufacturing such a cable, and some of the prior art techniques cause, rather than solve, problems at these extended frequency ranges.
Pretwisting (U.S. Pat. No. 5,767,441—Brorein '441) was introduced to eliminate the random effect of conductor to conductor spacing, but It is to appreciated that this arrangement also generates its own problems in the new frequency ranges of interest. The random conductor to conductor spacing caused undesirable effects in the electrical parameter of return loss. Although this technology is widely used in the data communication cable industry, it was discovered that the pretwisting of the conductor also results in degradation of electrical properties, such as return losses, due to conductor deformation effects. Those effects are now visible in the extended frequency range of interest.
Bonded pair technology (U.S. Pat. No. 6,222,129—Siekierka et al. '129) is a technology which controls the return loss parameters of a twisted pair by maintaining the conductor to conductor spacing. The main advantage of bonded pairs is to prevent the need for pretwisting of the conductor. However, such bonding does not control the spacing of the wires in the pair to pair shield or to an overall cable shield, so other means must be employed to establish and control the electrical properties defined by the interaction of the pairs to the cable shield components.
For non-shielded pairs, tightly wrapping or coating the two wires of a pair with a dielectric material is one technique for establishing and maintaining the mechanical integrity of the pair.
With respect to category 8 cables, it is to appreciated that such cables increase the frequency of operation for category cables to 2 GHz or more. This change reduces the electrical wavelength in the cable so that mechanical perturbations in the cable are longer than the electrical wavelength.
Until Category 8 cables, the periodicity length of manufacturing operations is longer than the electrical wavelength. However, this changes with Category 8 cables.
When frequencies greater than 2 GHz are required, even shorter periodicity lengths are required and this, in turn, substantially increases the electrical delay and insertion loss effects.
Other cable designs have performance above 2 GHz, but the industry desires to have a cable construction that is very similar to existing Category 6 and 7 constructions. Such similarity of construction allows ease of adapting cable connectors, termination practices, installation ease and familiarity, etc.
The periodicity length in the cable is accompanied by an insertion loss notch at the frequency corresponding to the length. A return loss spike accompanies the insertion loss notch. With conventional equipment, even equipment with updated design and controls, the periodic perturbations cause insertion loss and return loss results that do not meet the cable specifications.
The inventors have discovered that the root cause for the electrical problems result from one or more minor inconsistencies in the mechanical structure of the cable, over its entire axial length, which are normally caused by the associated manufacturing equipment, e.g., cabling of the cable core assembly during manufacture of the cable.